The present invention relates to the manufacture of valves for internal combustion engines, and more particularly, to the alignment of the valve head relative to the valve stem.
Poppet valves for internal combustion engines are conventionally made by forging, after which the valve head must be aligned relative to the valve stem to facilitate finish machining operations.
In the prior art it has been common practice to align the valve head relative to the valve stem by rolling the valve stem between a pair of flat dies, usually with one being stationary and the other reciprocating back and forth relative to the stationary die. With this arrangement, the alignment of the valve head was accomplished by positioning a straight head straightening bar on the stationary die, thus engaging the surface of the valve head and aligning it relative to the stem during rolling of the stem. Although this provided a generally satisfactory alignment between the valve head and the stem the use of a reciprocating die meant that only one valve could be processed at a time. In addition, because of the difference in the diameters of the valve stem and valve head, the surface speed of the valve head is much greater than that of the stem, resulting in a skidding or rubbing action between the surface of the valve head and the stationary head straightening bar. Such a rubbing action resulted in wear of the surface of the straightening bar and/or of the valve head, and at the same time, the frictional force between the rubbing surfaces imparted a torsional force to the valve head, relative to the valve stem.
A more recent development in the prior art has been the use of a piece of equipment such as a rotary die thread roller where the thread dies have been replaced by a set of dies including a stationary die having an arcuate die surface and a cylindrical, rotating die having an external die surface, with the valve stem being rolled between the external surface of the rotating die and the arcuate surface of the stationary die. As in the previous description of the prior art, this apparatus utilizes a head straightening bar positioned on the stationary die. The use of such a rotary die increases the rate of production, as the die rotates continuously in one direction and additional valves may be positioned between the die surfaces while the previous valve is still being rolled. However, there still remains the problem of the rubbing action between the surface of the valve head and the surface of the straightening bar.